Stress limiter for electronic timepiece indexing mechanism



June 28, 1966 w. o. BENNETT ETAL 3,257,794

STRESS LIMITER FOR ELECTRONIC TIMEPIECE INDEXING MECHANISM 2 Sheets-Sheet l Tiai A.

Mam/u 0 :94

Filed Dec. 29, 1964 L l 4 T W 0 w @L m5 Q L W w Q l wwww W W W; W N z W Z wwwu Z fl w w 5% 4H June 1966 w. o. BENNETT ETAL 3,257,794

STRESS LIMITER FOR ELECTRONIC TIMEPIEGE INDEXING MECHANISM Fxled Dec 29 1964 2 Sheets-Sheet 2 INVENTOR5 Mwnn 0 66/Y/Y677 w Mum? 3,257 794 STRESS LIMITER FOR EIZECTRONIC TIMEPIECE INDEXING MECHANISM William 0. Bennett, Bayside, N.Y., and William W. Mutter, Paramus, N.J., assignors to Bulova Watch Company, Inc., New York, N.Y., a corporation of New York Filed Dec. 29, 1964, Ser. No. 421,786 7 Claims. (Cl. 58-23) This invention relates generally to horological instruments of the electronic type wherein the vibratory action of an electronically-actuated tuning fork or other form of high-frequency vibrator is converted into rotary motion by a ratchet and pawl mechanism. More particularly, the invention deals with a stress limiter adapted to prevent such mechanisms from being rendered inoperative by reason of shock or other abnormal stress-producing effects.

In the Patent 2,971,323 of Hetzel, there is disclosed an electronic timepiece including a tuning fork having a relatively high frequency and a battery powered transistorized drive circuit to sustain the vibratory motion of the fork. The reciprocating motion of the fork is transformed into rotary motion by means of a ratchet and pawl mechanism whose drive pawl or index finger is attached to one tine of the fork, the pawl engaging and advancing a ratchet wheel which drives a gear train for operating time indicators.

In the co-pending application Serial No. 302,956, filed August 19, 1963, by Bennett, Mutter and Van Haaften, and now issued as Patent No. 3,184,981, there is disclosed an improved form of a motion converter in which the ratchet wheel is caused to advance only one tooth for each forward stroke of the index finger attached to the tine, regardless of minor variations in the length of the stroke arising from changes in the amplitude of fork vibration. This is accomplished by means of an auxiliary pawl or click attached to the framework or pillar plate of the timepiece, this pawl engaging the ratchet wheel at a position relative to the index finger at which the phase between the finger and pawl is several ratchet teeth plus one-half tooth.

In the co-pending application of Hetzel, Serial No. 295,406, filed July 16, 1963, and now issued as Patent No. 3,167,905, there is disclosed an electronic timepiece in which index fingers are attached to both tines of a tuning fork, which fingers reciprocate in phase opposition and engage and alternately advance a ratchet wheel to provide a balanced operation.

The index fingers or pawls used in the various timepieces disclosed in the above-identified patent and pending applications are usually formed of flat spring material. Under certain abnormal operating conditions, these resilient elements are caused to buckle, and if they are overstressed to a point beyond acritical or limiting strain value, the element will be permanently deformed and the mechanism thereby rendered defective or inoperative.

For example, when the timepiece is subjected to a sudden shock, the tine to which the pawl is attached may be caused to swing abruptly toward the ratchet wheel to a significantly greater extent than is ordinarily encountered during normal vibration. n the other hand, because of its inertia, the index wheel will remain approximately in its normal position. The index finger, which extends between a ratchet tooth on the wheel and the point of attachment on' the tine, is therefore subjected to an exceptionally heavy stress causing it to buckle. This produces a severe bending of the index finger in the region adjacent the point of attachment to the tine. If the resultantstrain imparts a permanent bend to the finger,

the pawl will not recover its original form and can no longer engage the ratchet teeth on thewheel. Consequently, the motion of the fork will not be transmitted to the wheel, and the timepiece is inoperative.

Accordingly, it is the main object of this invention to provide a, pawl-and-ratchet motion converter for a vibratory member in a timepiece, which converter incorporates a stress limiter acting to prevent inordinate'bending and deformation of the pawl near its point of attachment to the vibratory member.

More specifically, it is an object of the invention to provide a stress limiter which is attached to the same post or point of connection from which the pawl extends and which is so positioned relative to the pawl as to restrict the bending thereof only when the pawl curvature assumes a radius approaching a critical value.

An important aspect of the invention resides in the fact that the stress limiter in no way interferes with the normal operation of the pawl and comes into play only when the stress imposed on the pawl is excessive and conducive to permanent deformation, the pawl otherwise not being engaged by the limiter.

Also an object of the invention is to provide an effective stress limiter for a timepiece which is inexpensive and which may be readily installed to render the timepiece shockproof. Thus it becomes possible to use the timepiece or horological device in military environments or in other applications which subject the device to unsual shock forces precluding the use of the more delicate conventional timepieces.

For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, where FIG. 1 is a perspective view of a tuning fork and motion converter of the type disclosed in said co-pending applications;

FIG. 2 separately shows the motion converter;

FIG. 3A shows the normal relationship of the index finger and the ratchet wheel in the motion converter;

FIG. 3B shows the relationship which arises from excessive stress in abnormal operation;

FIG. 30 shows the permanent deformation resulting from such excessive stress;

FIG. 4 is a sit-ress strain diagram;

FIG. 5 shows one embodiment of a stress limiter in accordance with the invention;

FIG. 6 illustrates a second embodiment of a stress limiter;

FIG. 7 illustrates a third embodiment of a stress limiter;

FIG. 8 illustrates a fourth embodiment of a stress limiter;

FIG. 9 illustrates a fifth embodiment of a stress limiter;

FIG. 10A illustrates a preferred embodiment of a stress limiter with the index finger in its normal position;

FIG. 10B illustrates the operation of this limiter when the index finger is stressed; and

FIG. shows the recovered position of the index.

finger when the stress is removed.

The basic motion transformer Referring now to FIGS. 1 and 2, there is shown a timepiece of the type disclosed more fully in the above-identi-fied copending applications and patents, including a I to cause the wheel to reverse direction.

or bearings and its timekeeping action is therefore relatively independent of the effects of friction.

Tuning fork is provided with a pair of flexible tines 14A and 14B interconnected by a relatively inflexible base 15, the base being provided with an upwardly extending stem 16 secured to the pillar plate or the framework by suitable screws. The central area of the pillar plate is cut out to permit unobstructed vibration of the tines.

The tuning fork is electromagnetically actuated through an electronic circuit of the type disclosed in the abovenoted Hetzel patents, including a magentic element 17 secured to the free end of tine 14A, and a magnetic element 18 secured to the free end of tine 14B. The manner in which the fork or other vibratory member is sustained in vibration forms no part of the present invention. The vibratory motion of the tuning fork is converted by motion transformer 13 into rotary motion. This transformer is constituted by a ratchet and pawl mechanism operated by the tuning fork to drive an index wheel W having ratchet teeth 19 thereon. -In .a working embodiment wheel W is provided with a large number of teeth (300) and a diameter of only 95/1000 of an inch, the length of each tooth being 8/ 10,000 of an inch.

Index wheel W acts as the actuator for rotary movement 11, and it is therefore intended that this wheel be advanced by the vibratory fork at a constant rate. This is effected by means of the main pawl or index finger F,

one end of which is secured to a post 20 projecting laterally f-rom tine 14B.

-Index finger F is in the form of a light leaf spring and carries a tip 21 which may be of precious or semiprecious stone, such as sapphire. The tip engages the ratchet teeth 19 of index wheel W so that the oscillations ,of the tine transmit turning impulses to the wheel. The

shaft of the wheel is provided with a pinion 22 which intermeshes with the first gear in the gear train 11.

Operating in conjunction with index wheel W is an' locked by locking screw 26. Arm 24 pivots about screw 27. In this way the point at which auxiliary pawl 23 engages the wheel W may be adjusted relative to the point of engagement of the index finger.

The index finger and pawl are both tensioned downwardly, the jewelled tips thereof being parallel with the teeth of the index wheel. The tension is such that when the finger is retracted,'there is sufiicient reverse torque This back-up, however, is arrested by the pawl which is phased several teeth plus one-half tooth from the finger and is positioned in advance thereof in the direction of wheel rotation. It would not be practical to maintain an exact amplitude for vibrations of the tuning fork in a wrist timepiece and the operation of the motion transformer is such that this is not necessary.

It will be noted that the spring forces on the index finger F and pawl 23 not only hold them in firm contact with the indexwheel W but they also exert a torque on this wheel, in the direction opposite'to its forward motion. This torque causes the index wheel to back up during the first portion of the return'stroke of the index jewel, until it is engaged by the pawl jewel. This torque is the result of the geometry of the system and is similar to the draw in a conventional escapement' which tends to hold the pallet fork against the banking pin.

The efiect of stress on the motion transformer shown in FIGS. 1 and 2, or to any other form of motion transformer wherein one or more pawls or index fingers are coupled to a vibratory member and engage an index wheel or rack device having ratchet teeth. For purposes of illustrating the eifect of stress in its simplest possible form, we shall in FIGS. 3A, 3B and 3C, consider normal and abnormal conditions which prevail during operation of tine 14B in FIG. 1, which is coupled by index finger F to ratchet wheel W. It will be appreciated however that the invention is not limited to this application.

It will be seen in FIG. 3A that one end of index finger F is inserted into a hole in post 20 and is held therein by a tapered pin 28. This hole may in practice be round, square-shaped, or in any other form. The attachment of the finger to the post may also be accomplished by riveting, welding or any other known means.

The jeweled tip 21 of the finger F is received by a ratchet tooth 19 in the wheel W. In normal operation of the tuning fork, as the tine 14B swings in the direction toward index wheel W, the finger tip pushes against the ratchet tooth to thereby turn the wheel, and when the tine swings back, the finger tip is retracted and drops into the next tooth. It will be noted that in normal operation, little flexing occurs in the finger.

We shall now, in connection with FIG. 3B, consider an abnormal operation. At the instant of a heavy shock, tine 14B is caused to swing sharply from its initial position, shown in broken lines, to the position to the left thereof, shown in solid lines. The index wheel W, which is coupled to the gear-works, because of its inertia, does not turn in response to this :blow but remains approximately at its initial position. As a consequence of the stress imposed by the tine on one end of the finger, whose other end is effectively held by the ratchet wheel, the index finger is forced to buckle or bow outwardly to an extent determined by the degree of tine swing. The resultant bending of the finger is greatest in the region adjacent the point at which the finger is attached to post 20.

In orderto appreciate the eifect of this action on the finger, reference is now made to the stress-strain diagram illustrated in FIG. 4. Stress'is the intensity of force imposed on a unit area. In the case of index finger P, we are dealing with a stress on an elastic member resulting from a compressive force imposed thereon by the tine. Strain is the measure of the amount of deformation the finger undergoes when it experiences stress.

For spring materials which recover their form after the stress thereon is released, the plot of stress (S) versus strain (E) is practically a straight line. Hence it will be seen on the diagram that as the compressive stress is increased, strain is directly proportional to stress. However, above a limiting or critical value L, which is the limit of proportionality, the relationship is no longer pro- Thus, for stresses up to value L, when the the finger will take on a permanent set, as shown in FIG. 3C. As a result, the finger tip no longer lies in engagement with the ratchet teeth and the timepiece is rendered inoperative.

Measurements on a variety of materials suitable for index fingers have indicated that for values E the degree of strain at the proportional limit varies from approximately 0.75% to about 1.5%, depending on the material being tested. When a straight member having a thickness (t) is bent into a circular shape having a radius (R), the outermost surface of the member is strained in tension. Assuming that the neutral axis (the unstrained .plane of the circularshape) is in the center of the section, it can be readily demonstrated that the elongation (Al/l) at the outermost surface is expressed by the following equation: 1

. safe curvature.

wherein: Z is length of finger, t is thickness of finger and R is radius of curvature of bent finger.

In such instances, the neutral axis lies at a radius slightly less than the radius of the center of the section. The thickness of the index finger in one working embodiment of a timepiece is .00055 of an inch. Inserting this value in the above equation, together with the minimum observed value (0.75%) for E for metals suitable for the index finger, the resulting critical value for R then becomes .037 inch. Hence in this specific example permanent deformation can be avoided if the index fingers can be constrained where it emerges from the post to which it is attached so that it cannot as a result of buckling, bend with a radius for curvature smaller than a radius of .037 inch.

Forms of the stress limiter Referring now to FIGS. 5 to 9, five different versions of a stress limiter in accordance with the invention are shown, each of which acts to restrict the bending of the index' finger F in the region where it leaves post 20 to a radius never smaller than R in the outward direction.

Such bending, as noted previously, occurs as a result of shock, and the limiter, by constraining the degree of bending prevents permanent deformation of the finger. It is to be noted that the smaller the radius, the greater the curvature of the bend, hence a larger radius avoids deleterious deformation.

, In FIG. 5, the stress limiter is in the form of a nose member SL welded or integral with the post 20, from which the index finger F extends, the nose projecting in the direction of the ratchet wheel but having an under surface which curves away from the linear .axis of the finger. This curved surface of the limiter is engaged by the finger, as shown in dotted lines, only when the finger bends outwardly so that the curvature of the finger in the region adjacent the post has a radius the value-of which cannot be smaller than the critical radius R at which'a permanent deformation occurs.

It will be apparent from FIG. 5, as well as in the succeeding figures, that the same type of stress limiter could be used to prevent inward bending of the index finger as well as outward bending, for the finger is capable of buckling in either direction. In practice, however, it has been found that when the finger buckles inwardly, as a result of heavy shock, the jewel tip is forced over the top of the index wheel teeth, which are very shallow, and therefore straightens out before the buckling is suflicient to bring about a significant bend inwardly at the point of attachment to the post. Hence it is not usually necessary to install a stresslimiter on the underside of the finger, although in some instances, this may be desirable.

In FIG. 6, the stress limiter SL is in the form of a nose, as in FIG. 5, having an extension which wraps about and clamps onto the finger post 20 whereby the limiter may .be installed without welding. The operation is otherwise the same as in FIG. 5.

In FIG. 7, the limiter SL takes the form of a curved bar having a straight extension received in the hole in the post 20 and held therein with the index finger by the tapered holding pin. The curvature of this bar is the same as the nose curvature on SL and SL and serves the same function.

In FIG. 8, limiter SL is a hook-shaped bar whose tip I is positioned to arrest further movement of the index finger when the finger assumes the limit of its safe curvature. The bar is held in the post by the pin, as in the case of FIG. 7. a

In FIG. 9, limiter SL is merely a pillar which is mounted on the framework of the timepiece at a position to intercept the finger when it assumes the limit of its It is to be noted that the limiter need merely engage the finger at a point thereon, rather than along a portion thereof in order to prevent excessive bending in the region adjacent the point of attachment.

Preferred form of stress limiter Referring now to FIGS. 10A, 10B and 10C, there is shown a preferred embodiment of a stress limiter SL in accordance with the invention. The limiter is constituted by a nearly straight and rigid bar member, one end of which is secured by the holding pin 28 in the post 20 from which the index finger also extends. The length of the member SL is about one-third the active length of the index finger from its point of attachment to the post. The rigid member has a slight bend therein so that it is angularly displaced from the finger and does not touch the finger under normal operating conditions, as shown in FIG. 10A.

When the index finger is bent, as shown in FIG. 10B, in response to a stress-producing shock, it is intercepted by the free end of the rigid member SL at a point preventing the finger from assuming an excess curvature. Thus when the stress is relieved, the finger straightens out and returns exactly to its previous position, as shown in FIG. 10C. In all instances, therefore, the stress limiter prevents the index finger from bending during shock at a radius less than the minimum established by the physical properties and thickness of the finger material.

While several embodiments of stress limiter in accordance with the invention have been shown and described, it will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit of the invention as defined in the annexed claims.

What we claim is:

1. In a horological device having a vibrating timekeeping standard, a gearworks and a motion transformer intercoupling said standard and said gearworks to convert the vibratory motion of the standard into rotary motion to drive said works, said transformer comprising:

(a) an index finger attached at one end to said standard and reciprocating therewith,

(b) a ratchet wheel whose teeth are engaged by the other end of said finger to effect turning of said wheel, said wheel being operatively coupled to said gearworks, said finger being formed of a material having a predetermined thickness and being caused to bend when said device is subjected to a shock imposing a stress on said finger, and

(c) a stress limiter to restrict the bend of said finger in the region adjacent its point of connection to said standard within limits preventing permanent deformation of said finger.

2. In a horological device having a vibrating timekeeping standard, a gearworks and a motion transformer intercoupling said standard and said gearworks to convert the vibratory motion of the standard into rotary motion to drive said works, said transformer comprising: (a) an index finger attached at one end to said standard and reciprocating therewith, (b) a ratchet wheel whose teeth are engaged by the other end of said finger to effect turning of said wheel, said wheel being operatively coupled to said gearworks, said finger being formed of a metallic material having a predetermined thickness and being caused to bend when said device is subjected to shock imposing a stress on said finger, and (c) a stress limiter to prevent said finger from excessive bending in the region adjacent its point of connection to said standard, said limiter acting to restrict said bending to a radius of curvature which is less than a predetermined value established by the physical properties and thickness of said finger.

3. In a horological device having a timekeeping standard in the form of a vibrating tuning fork, a gearworks and a motion transformer intercoupling said fork and said gearworks to convert the vibratory motion of the fork into rotary motion to drive said works, said transformer comprising:

(a) an index finger attached at one end to one tine of said fork and reciprocating therewith,

(b) a ratchet wheel whose teeth are engaged by the other end of said finger to effect turning of said wheel, said wheel being operatively coupled to said gearworks, said finger being formed of a material having a predetermined thickness and being caused to bend when said device is subjected to shock imposing a stress on said finger, and

(c) a stress limiter to restrict the bending of the finger within a limit preventing permanent deformation thereof.

4. In a device as set forth in claim 3, wherein said finger extends from a post mounted on said tine, and said limiter is constituted by a nose member attached to said post and projecting forwardly therefrom toward said ratchet wheel, said nose member having a curved undersurface which is engaged by said bent finger only at said limit.

5. In a device as set forth in claim 3, wherein said finger extends from a post mounted on said tine, and said limiter is constituted by a bar attached to said post and projecting forwardly therefrom, the end of said bar being curved to provide an abutment which engages the bent finger at said limit.

6. In a horological device having a vibrating timekeeping standard in the form of a tuning fork, a rotary gearworks and a motion transformer intercoupling said fork and said gearworks to convert the vibratory motion of the standard into rotary motion to drive said works, said transformer comprising:

(a) an index finger attached to a post projecting from a tine of said fork and reciprocating therewith,

(b) a ratchet wheel, whose teeth are engaged by the other end of said finger to effect turning of said wheel, said wheel being oper-atively coupled to said gearworks, said finger being formed of a material having a predetermined thickness which is caused to bend when said device is subjected to shock imposing a stress on said finger, and

(c) a stress limiter to prevent said finger from bending in the region adjacent its point of attachment to said post with a radius of curvature which is less than a predetermined limit established by the physical properties and thickness of said finger.

7. In a device as set forth in claim 6, wherein said limiter is in the form of a substantially straight and rigid bar attached tosaid post and extending therefrom in the direction of said finger, the forward portion of said bar being bent to provide a stop which engages said finger only when it bends to a degree approaching the said limit.

No references cited.

LOUIS J. CAPOZI, Primary Examiner. 

1. IN A HOROLOGICAL DEVICE HAVING A VIBRATING TIMEKEEPING STANDARD, A GEARWORKS AND A MOTION TRANSFORMER INTERCOUPLING SAID STANDARD AND SAID GEARWORKS TO CONVERT THE VIBRATORY MOTION OF THE STANDARD INTO ROTARY MOTION TO DRIVE SAID WORKS, SAID TRANSFORMER COMPRISING: (A) AN INDEX FINGER ATTACHED AT ONE END TO SAID STANDARD AND RECIPROCATING THEREWITH, (B) A RATCHET WHEEL WHOSE TEETH ARE ENGAGED BY THE OTHER END OF SAID FINGER TO EFFECT TURNING OF SAID WHEEL, SAID WHEEL BEING OPERATIVELY COUPLED TO SAID GEARWORKS, SAID FINGER BEING FORMED OF A MATERIAL HAVING A PREDETERMINED THICKNESS AND BEING CAUSED TO BEND WHEN SAID DEVICE IS SUBJECTED TO A SHOCK IMPOSING A STRESS ON SAID FINGER, AND (C) A STRESS LIMITER TO RESTRICT THE BEND OF SAID FINGER IN THE REGION ADJACENT ITS POINT OF CONNECTION TO SAID STANDARD WITHIN LIMITS PREVENTING PERMANENT DEFORMATION OF SAID FINGER. 